Author Topic: Another HP 1741A repair  (Read 1197 times)

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Offline Zamm

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Another HP 1741A repair
« on: April 18, 2017, 10:39:04 AM »
I'm new to electronics after having my interest piqued while being introduced to Arduinos and programming at large. Wanted to get deeper in the discrete components and build out some minimal skills to do diagnostics and testing. In line with that, I have slowly accumulated a few basic instruments, and had been keeping my eye out for a scope. As luck would have it, I saw one posted locally on Craigslist, and I went over to pick it up untested for $60. Luckily for me, it also included the hard copy service manual, which I have been fawning over as I admire the schematics.

Quick summary of the scope:
Good overall condition.  A little grimy on the outside and on the front panel, but inside is very clean. A review of the knobs and buttons reveal that all actuate well, except for the "line" (on/off). Luckily for me it is stuck in the ON position, so I am turning the scope on via the power cord.

First power on, I got a single point cycling across the screen. I probed the 1v cal and saw a good, flat square wave.

Then things got a little funky. I was waiting for one of the cheapo assemble it yourself signal generators to show up in the mail, so I didn't really have anything fun to do with the scope. But for the next couple of days I couldn't see anything on the screen. When I would turn up the beam intensity, the left side of the screen would start bleaching out green. Then I would press the beam find, I would suddenly see a dot on the left half of the screen. My gut was telling me I had an issue with my horizontal.

This afternoon, I got home and decided to start probing the horizontal controls as it went into the gun. At some point I looked at the display and saw I had proper functionality, but that would come and go. Sometimes it would decide not to show anything, and I would switch between displays and time/div settings and poke at the horizontal control boards and by luck it would turn back on. I'm attaching some pictures from today to show that I encountered.

First set is a sine wave, then the beam find while the function was applied. I'd be interested to hear from folks if I should expect to see the wave come back over itself on the right side as it shows.
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Second set is the cal square wave, then the beam find while the cal was applied. Similar question as above.
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The third set is what I would encounter when I would be unable to see a signal. I would apply the cal wave, not see anything, and then press the beam find and see a vertical line. My assumption is that it proves that I am getting good vertical, but intermittently not getting horizontal.
5" border="0

Lastly, at very small increments of time/div, I am seeing what I would best characterize as back trace. It isn't happening all the time, and the back trace is generally softer than the main trace, but I gather its not supposed to be there.
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As far as general operations, the trace strikes me as a bit faint. I will change the time/div and, especially when they are very small units, it will take a bit for the line to come into focus, or may require that I apply additional brightness to make it out.

I'd be interested to know if folks had any gut instincts on the matter before I start digging around randomly (which is about the best outcome, despite having access to the manual). My plan is to review the schematic and try to plot a course to diagnose the issues further. I have also procured deoxit to clean the time/div contacts. I tested the LVPS and am seeing good measurements per the manual. I don't have the probe needed to dive into the HVPS.




 

Online xrunner

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Re: Another HP 1741A repair
« Reply #1 on: April 18, 2017, 10:50:34 AM »
Welcome to the forum and nice first post.

Well, my first advice,having worked on a lot of older test gear - clean the s**t out of it. Re-seat every single connector inside (with the power off  :)), but also clean the pins. Clean pots with something like deoxit. It will do wonders for it. I bet it starts to act a whole lot better after doing that.  :-+
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Offline helius

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Re: Another HP 1741A repair
« Reply #2 on: April 18, 2017, 11:29:42 AM »
These storage scopes are very interesting but can also be confusing. First see if it works correctly in CONV (non-storage) mode.
 

Offline lowimpedance

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Re: Another HP 1741A repair
« Reply #3 on: April 18, 2017, 12:01:47 PM »
Indeed set it to conventional mode and check trace. And being a storage type the trace is quite a bit dimmer at faster time base speeds than its cousin the 1740a.

As suggested a good clean, (looks like its well and truly due for one :D), all major assemblies are edge connected but there are still quite a few wires to pins too so take plenty of photos to help re assembly.
A couple of other points, the grease on the time base switches dries out and causes all sorts of issues with alignment and thus operation. Stripping and cleaning is the only cure. Lastly the most common issues are generally the jelly bean parts like the carbon comp. resistors drifting high causing biasing failures in the DC coupled vertical and horizontal amps , and of course power supply caps / transistors , dry joints etc. The hybrids are quite robust and mostly not the cause of failure.

Anyway first step ............ a good clean   :)
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Offline edpalmer42

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Re: Another HP 1741A repair
« Reply #4 on: April 18, 2017, 12:21:46 PM »
Look carefully at the small transistors and the ICs in round metal cans.  At first glance, you'll think they're soldered in.  They're not!  Each lead is in its own seperate socket.

My 1744A storage scope quit one day and a few quick tests showed that the problem was in one of the power supplies.  I looked at the 723 regulator in the round metal can (TO-100).  I noticed the goofy little sockets.  I wiggled the IC.  Problem solved!

Ed
 

Offline Zamm

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Re: Another HP 1741A repair
« Reply #5 on: April 18, 2017, 12:29:29 PM »
Thanks for the first round of replies, I really appreciate your input!

Re: Conv mode - I was able to switch over to it and it provided a trace without major issue. I would note that it comes across as quite faint, especially when I press find beam as a comparison. The focus on the find beam is very tight and the dot or line is what I would describe as "sharp" or "piercing". A far cry from what the display is when I let off the beam find, the line is much thicker and much fainter (in full light I might not even see it).

Re: cleaning - I have been somewhat intimidated to start taking the whole thing apart. I know that with enough pictures and good parts management I'll get it back eventually, but still. At this point though, the thing keeping me from diving in further is figuring how to take the knobs off the pots. They are all secured with small allen bolts, but I don't know what size they are. I bought a cheapo set of allen keys in both standard and metric, and a few "fit" but when I try to turn, I'm afraid they are rounding/stripping. I don't know if anyone has any specific tips here, I'm certainly afraid to need to drill them out.

Re: cleaning, cntd - Anyone have suggestions on how to use deoxit? I bought deoxit D5 and some no-lint applicators for big parts, but as a general rule can I just go hog wild on spritzing the parts or do I need to approach it with care?
 

Online tautech

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Re: Another HP 1741A repair
« Reply #6 on: April 18, 2017, 12:50:09 PM »
There is much you can do without knob removal if the hex set screws are too tight for your comfort.
They need be quality hex keys (all imperial, NOT metric) and must be a very snug fit, then they will move with a "crack".
I checked a 1740 manual for the hex key sizes but they are not listed but there has been some discussion in the past that I think you can find in 1740 threads.

The timebase PCB's are each on the timebase shaft that is retained with retaining collars on the timebase shaft. Each collar has a set screw and when they are all loosened the timebase shaft pulls out the front. This allows several PCB's to be removed and thereby creates a lot of room and gives you the chance to check the interconnects which BTW are known to crack solder joints. Check them all especially the 3 way one which IIRC links 3 PCB's and is without componentry.
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Offline David Hess

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Re: Another HP 1741A repair
« Reply #7 on: April 18, 2017, 01:38:47 PM »
I agree with helius, learn to use it without the storage functions first.

Analog storage oscilloscope CRTs are not as bright and sharp as non-storage CRTs.

Besides cleaning the various switches and potentiometers, check the low voltage power supply outputs for level and ripple.

Based on your photographs, it looks to me like it is working fine.
 

Online james_s

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Re: Another HP 1741A repair
« Reply #8 on: April 20, 2017, 06:13:46 AM »
When it comes time to clean it, I've found that a product called Oil Eater is fantastic for removing that icky grime that builds up on test equipment. I took the panel apart on a filthy TDS340 I got and after scrubbing it in the sink with Oil Eater, rinse, dry and reassemble it looks brand new.
 

Offline oldway

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Re: Another HP 1741A repair
« Reply #9 on: April 22, 2017, 02:34:23 AM »
....
I checked a 1740 manual for the hex key sizes but they are not listed but there has been some discussion in the past that I think you can find in 1740 threads.
I measured the key size, it is 0.05 inch
 

Offline Zamm

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Re: Another HP 1741A repair
« Reply #10 on: April 23, 2017, 12:51:17 PM »
First off, thanks all for the thoughts and inputs along the way. I will have to give the degreaser a try, since washing the top and bottom panels of the case with dish soap didn't yield the results I wanted.

I finally had some time today to dig back through the scope and pulled out the LVPS. As with many videos and other folks' experience, there were cold solder joints on a few of the plugs, but otherwise no issues on that board. While I had it off, I was able to get the line switch to work again (previously stuck in ON luckily). A little bit of deoxit in the top of the switch with a lot of button actuation. The first deassemble/reassemble procedure boosted my confidence to work with it further.

I've been spending additional time with the schematic diagrams to really get a feel for what is going on, and teaching myself how to read it. While I may be lost in the theory of why, I can at least generally trace that what, and follow the steps to test at critical points.

Given that I have succeeded in limiting my issue down to an intermittent (ugh...) issue with the horizontal. I am occasionally losing all sweep, and the beam will focus about a third of the way in from the left of the CRT. Occasionally I will have luck and it will sweep like normal and give me the pretty signal pictures I'm expecting. [Please excuse/correct me on the term "sweep" if thats not the technical term I should use here]. Based on my reading, I'm going to try to work on the horizontal preamplifier circuit board as well as the horizontal sweep assembly that feeds it.

I just realized that my cheapo ebay DIY XR2206 function generator has a 600 ohm impedance, while the diagnostics say to "connect pulse generator 50-ohm output to model 1741a channel a input connector." Anyone know if that is going to cause issues?
 

Offline tggzzz

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Re: Another HP 1741A repair
« Reply #11 on: April 23, 2017, 06:18:52 PM »
I just realized that my cheapo ebay DIY XR2206 function generator has a 600 ohm impedance, while the diagnostics say to "connect pulse generator 50-ohm output to model 1741a channel a input connector." Anyone know if that is going to cause issues?

It is fine for getting things going, but it will be useless for calibration due to amplitude errors, probably a poor rise/falltime, and probably ringing.
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